Find or buy poster paper. Any color will do, but neon colors tend to stand out more on camera. It also depends on what materials you have around. White poster paper is the most common for wrestling signs, but you can be versatile with what you use to draw on it. Neon paper should have thick, dark lettering or pictures, so make sure you have enough dark drawing materials. Black poster paper will need a white or neon marker to be seen from afar. Make sure the colors you use stand out on the poster board; look from afar, not just up close.
State your opinion about what wrestler you like or dislike. Simply writing the wrestler's name will let them know you support them. If you have a group of people, try dividing the name onto a few posters so the wrestler will know a number of people support him. On the other hand, if you write a wrestler's name with an insult attached, everyone will know your opinion is that you are mocking that wrestler.
Make it funny. There are many types of jokes that can fit into a few words--for instance, deliberately misspelling a wrestler's name so it looks like another word or writing the wrestler's catchphrase and adding why it's wrong or should be different. Wrestleview.com has a look into some of the funnier wrestling signs.
Draw what you know how to draw. If you can draw, try adding a sketch of a wrestler. The best or most detailed sketches often end up on television. If you can't or don't want to draw, you can also stick pictures onto your poster.